Suggest Treatment For Rashes Under The Breast

I have had a rash or discoloration under my breasts for about a year and a half. It started under one breast, and spread under the other one. It started as small round spots, and progressed to a more solid discoloration. It is not painful at all, and does not itch. It does look like it is spreading some more, going onto my breast and to my shoulder. I have been noticing some fatigue as well.